1. Field
Exemplary embodiments of the present invention relate generally to an image sensor and, more particularly, to a pixel biasing device for canceling a ground noise of a ramp signal from a ramp signal generation apparatus through a correlated double sampling (CDS) operation of a comparator.
2. Description of the Related Art
In general, an image sensor, such as a complementary metal oxide semiconductor (CMOS) image sensor, converts an analog signal (i.e., a pixel signal) generated from a pixel array into a digital signal (i.e., a pixel data) unlike a solid state pick-up device. An analog-to-digital converter (ADC) having a high resolution is used in the CMOS image sensor to convert the analog signal into the digital signal.
The CMOS image sensor may use a single ADC or a column ADC.
A CMOS image sensor with a single ADC uses a method for converting an analog signal outputted from a pixel array of all columns within a predetermined time using the single ADC, which operates at a high speed, and reduces a chip area of the CMOS image sensor. However, a CMOS image sensor with a single ADC consumes generally a larger amount of power because the ADC operates at a high speed.
A CMOS image sensor with a column ADC is implemented by disposing a single ADC at each column. Hence one disadvantage of a CMOS image sensor with a column ADC is that it generally requires a larger chip area. On the other hand, a CMOS image sensor with a column ADC consumes less power than a CMOS with a single ADC because each of the ADCs operate at a low speed.
A CMOS image sensor with a column ADC performs a CDS operation on an analog output voltage, that is, an output signal of a pixel array, stores a result voltage of the CDS operation, and provides a comparison signal for generating a digital code by comparing the stored result voltage of the CDS operation with a predetermined reference voltage (i.e., a ramp signal) in response to a ramp signal generated from a ramp signal generator.
As described above, since a conventional CMOS image sensor uses different voltages in a ramp signal generation apparatus and a pixel array, a pixel power noise and a pixel ground noise may occur in a pixel array. Also, a ramp power noise and a ramp ground noise may occur in a ramp signal generator. The pixel power noise, the pixel ground noise and the ramp power noise among the above-described noises may be canceled by various well-known techniques, but it is difficult to cancel the ramp ground noise, and a signal noise may occur due to the ramp ground noise.